I'm a junior developer, so I've no experience of mentoring, but I was a school physics teacher before starting this career. In my teacher training, I learned that the three questions anyone planning a lesson should ask are:
It seems to me like neither 1) nor 2) is completely clear here, but let's assume the answer to 2) is "Nothing about programming at all."
What do you want them to learn?
My guess is that you want them to become productive members of your team, but what does that mean? I figure a software developer has to know about the following things:
In addition to these, you'll also want them to develop the skills of analysing and solving problems.
Now, how does your choice of language affect any of these? Most of these concepts are either language-agnostic (a binary tree is a binary tree in any language) or their implementation is language-specific without being transferable (Is understanding Maven useful if you're using npm? What about if you're vendoring a Ruby gem?). The upshot of this is that if there's one language you work particularly with, it obviously makes sense to start with that. Focus on one idiom, one ecosystem, one API, and teach them how to write working and maintainable code.
But as you're asking this, I guess that doesn't describe your situation. So what then? I'm not sure I'd describe choosing between say, Python and C as a better or worse learning experience, but I'd say there are tradeoffs to be made.
A statically typed, compiled language will teach you computer science concepts that a dynamically typed, interpreted language will not. I'm pretty certain that discussions about pass by reference or by value, for example, would be completely opaque to me if I hadn't studied C(++) when I was first learning to program back when I studied physics and I just needed the computer to give me a damn number lots of times so I could draw a graph. Similarly, this makes you think about your code's journey from your text editor to an executable. I never learned C well enough to really worry about garbage collection, as the scripts I wrote ran so quickly that the OS would sort that for me, but these concepts have been useful.
By contrast, progress will in other respects be slower in these languages. Partly because you're learning computer science as well; partly because all those variable declarations put an extra strain on your working memory until they become second nature; and partly because whacking code into the interpreter and hitting enter gives much more rapid feedback than linking, compiling, and running. (The importance of this feedback in the learning process shouldn't be underestimated, and a learner's working memory should be allocated more sparingly than RAM. On a related note, regarding your observations about learning on punchcards: I once wondered how much difference the ubiquity of soap dispensers or kitchen disinfectants actually makes to human health when we evolved just fine without them over a few hundred thousand years. My brother, who's a doctor, observed that life expectancy back in 1800 was 32. These advances matter.)
You can push these arguments as far as you like. I've often thought that C-style syntax is so ubiquitous you'll make up for any lost time as soon as you learn a second language- of the top 7 TIOBE languages, Python is the odd one out in terms of syntax (not that I know PHP). However, I've learned a huge amount from working on real live production code. It's one thing to read some guy tell you in a book to keep your functions short and DRY; it's quite another to actually be faced with a 200 line method and identify the 5 places you need to change it. If Python (or Ruby) means you can actually read and modify real non-trivial production code without causing a memory leak early in your career, there's probably a lot to be said for that.
The other aspect of a language to think about is paradigms. Javascript has C syntax and the browser console, but in spite of everything I've said so far I'd advise against learning it as a first language- not because of its dependence on global variables, or its ever-expanding ecosystem, but because Eric Elliot estimates that 99% of JS developers don't know what they are doing. Prototypal inheritance, first class functions, its approach to asynchronous programming, ... make JS a wildly different beast from most other commonly used languages, and it's best treated as such. When I first decided to make the career switch to programming, a friend advised me to learn Java- partly for the jobs market, and partly because you learn to write modular code (everything is a class, and one class per file).
Ultimately, no single language is going to teach you everything- C will teach you pointers, but not functional programming. Your "learning experience" is incomplete if it's limited to any language, and the optimal route through them probably differs both on the person learning and the goal they're working towards.